It's my last day of school in a couple of weeks and me and my friends want to make it a day to remember!! All the other years before us have come up with really orginal ideas like a washing line of bras and knickers! so we want to out do them all and make people rember us!
We already have quite a few ideas but they are not all that oringinal:

Writing a msg with weedkiller on the grass
Water guns
Water bombs with condoms
Spray painting the senoir girls mirrors black (they spend alll their time in there!!)
Rude helium ballons in the hall - they take ages to get down
Music playing from the roof
Putting fish behind the radiators
Glue the windows shut
Bubble bath in the toilets - the over flow!
Grass seed in the carpets
Glitter on teachers work and in draws
Swap board pens for permanate ones

alot of threads on this in the past
the favorate is the 3 sheep prank number 1 2 and 4

at my school we took personal attack allarms, tied them to helium ballons and let them go in all high celing rooms (about 5 places )
we also tied balloons to the cars of teachers we didnt like. no idea why but neither did they. the intimdation was there
just make sure things you do dont warrent criminal damage

We unhinged all the doors in the science department and hid them in the woods next door. We also hid all the clocks in the classrooms and put them in the ceiling. Oh yeah, and some people put talc all over teachers cars. Not sure why, but there we go.

There were loads of these threads last year, try searching for some more.

(Original post by dh00001)
at my school we took personal attack allarms, tied them to helium ballons and let them go in all high celing rooms (about 5 places )
we also tied balloons to the cars of teachers we didnt like. no idea why but neither did they. the intimdation was there
just make sure things you do dont warrent criminal damage

I like the idea of playing music from the roof! That would be sooo cool!
Also swapping the board pens for permenant ones would be pretty funny cuz that would be really annoying but wouldnt cause to much damage etc.

(Original post by dh00001)
alot of threads on this in the past
the favorate is the 3 sheep prank number 1 2 and 4

Last years 6th year left dead fish in the ceiling tiles of our common room (among various other surprisingly original things), it still smells even to this day. Not sure what we're going to do, we'll be struggling to outdo last year.

Quite a good one: get some brightly coloured powder paint from the art department, and put a big pile on a piece of tissue paper. Tape the tissue to the air intakes of the hand driers in the toilets. When someone sticks their wet hands under to dry them, they'll get covered in bright paint!

Last year some girls put the school's millenium statue (an unusual little thing which cost £20,000) on E-bay....and the headmistress' house (which is in the school grounds). Bit dubious about the legality of that though...
Another quite funny one was; put poppers (no not the drugs, the little cap things that go pop) under the toilet seats so that when people sit down they go off.
The most common one in my school is to use the gym pants though. If your school has regulation gym pants then there is so much you can do with them. In case you don't know what i'm on about, gym pants are giant umm.. well pants really, that girls (i don't know if there is a boys equivilant) wear with a t-shirt as an entire PE outfit, but they are hideous (my schools aregiant and maroon, hence the nick-name marooners). String them up to make very fetching banners, put them in local landmarks, tie them to other (or even better rival) school's railings...the list goes on!
I would reitterate the don't do any damage thing though, and also don't throw eggs. A nearby school threw eggs at some of the younger girls who go to my school, the egg shell cut one girl's face and eye -she ended up in hospital and still has a scar.